In fluid dynamics, DSV (Dynamic Stall Vortex) is known. The dynamic stall vortex is generated, for example, when an angle of attack of a wing to a flow of a fluid is oscillated across a static stall angle. In this case, even when the angle of attack exceeds the static stall angle, a dynamic lift does not decrease (a stall does not occur) but increases. At this time, the dynamic stall vortex is generated, and it is thought that a great dynamic lift is generated due to a negative pressure of this vortex.
However, when the angle of attack of the wing is increased to a certain degree or more exceeding the static stall angle, the dynamic lift, after reaching the maximum, rapidly decreases, resulting in a complete stall. At this time, the dynamic stall vortex is not generated, and therefore there occurs a state where no negative pressure due to the dynamic stall vortex exists.
As described above, the dynamic stall vortex generates a large dynamic lift and on the other hand becomes a cause of instability of the dynamic lift. Therefore, in technical fields using the dynamic lift to wings, such as an aircraft (a fixed-wing aircraft, a rotary-wing aircraft, or the like), a windmill, and the like, designs are generally made so that the angle of attack of the wing becomes sufficiently smaller than the stall angle in order to prevent the occurrence of a dynamic stall (in other words, in order to prevent the generation of a dynamic stall vortex).
However, if the generation of the dynamic stall vortex can be controlled, it is possible to perform various kinds of processes (for example, the application of a force to an object, the promotion of the mixture of gases) by utilizing characteristics of the dynamic stall vortex (a high non-steady negative pressure and so on).